1. Field of the Invention
The present invention relates to a polysiloxane-containing rubber composition having improved properties and a rubber compounding agent therefor.
2. Description of the Related Art
Rubber compositions comprising various rubbers and a filler such as silica are known in the art and are used, for example, as rubber compositions for tire treads having a low heat generation characteristic and superior in abrasion resistance etc. However, tire treads in which silica is formulated have a low rolling resistance and a good grip on wet roads, but there were the problems that the viscosity of the unvulcanized compound increases, the vulcanization is slowed, the kneading performance in the mixing falls, etc. and the productivity deteriorates. Various proposals have been made in the past to solve these problems, but unfortunately these are not practically satisfactory.
For example, it has been proposed to add diethylene glycol or a fatty acid (for example, see Gomu Kogyo Binran (or Rubber Industry Handbook), fourth version, pp. 517 to 518, published 1994), to add a metal salt of a carboxylic acid (for example, see Tire Technology International 1995, pp. 107 to 108), and to treat the silica in advance with silicone oil (for example, see Japanese Unexamined Patent Publication (Kokai) No. 6-248116). Further, the only way to deal with scorching during mixing and a fall in the kneading performance is to increase the number of times of mixture. Further, even when mixing carbon and silica, the practice is to either mix them separately or to increase the mixing time or frequency of mixture. None of these are unfortunately practically acceptable.
On the other hand, various metal oxides are used in many fields such as rubber, cosmetics, synthetic resins, coating compositions, adhesives, magnetic tapes. In most cases, since the metal oxides are added to organic materials, problems arise how the metal oxides are dispersed in the organic materials. For this purpose; various attempts have been made to solve these problems. For example, the treatment of the metal oxides with surface treatment agents has been proposed. JP-A-8-48910 discloses a method for treating a metal oxide with methyl hydrogen polysiloxane, in which the metal oxides are necessary for treating at a high temperature. JP-A-8-53630 discloses the treatment with an alkoxy silane, but this is not still sufficient.
On the other hand, rubber compositions comprising various rubbers and metal oxides such as silica, clay, talc are known and are used as, for example, rubber compositions for tire tread having excellent low heat build-up and abrasion resistance. However, when metal oxides are compounded, there are disadvantages, when compared in the case of carbon black, the modulus is decreased and the abrasion resistance is low. To solve these problems, the above-mentioned JP-A-6-248116 has been proposed. (i.e., silica is surface-treated under heating with, for example, a hydrophobic agent such as silicone oil comprising an organic silicon compound at, for example, 250.degree. C. for 1 hour. However, this method has a disadvantage of a short scorching due to heating at a high temperature.